The invention relates to a method for navigating a vehicle, in which a change in the vehicle""s direction is ascertained by integration of a signal indicating a speed of change in direction.
Such a method is used in a xe2x80x9cCarinxe2x80x9d navigation system, for example.
The demand for navigation aids has increased continuously in recent years. The aim here is to be able to indicate to the driver of a motor vehicle the best possible route for him to be able to reach the destination from a starting location. In this context, the overall task is divided into three parts, namely determining the position of the vehicle, route planning, and transmitting the route to the driver. The present invention relates to the first part, namely determining the position of the vehicle.
To determine the position of the vehicle, navigation satellites forming part of a global positioning system, or GPS, have been available for some time. Although the satellites can be used everywhere to determine a location or position with few restrictions, the accuracy which they are able to achieve is only within a range of 100 m. For inter-urban journeys, this may be sufficient in most cases. However, this xe2x80x9cinaccuracyxe2x80x9d is critical where the distance between roads is less than this magnitude, for example, in towns or at traffic junctions, where a number of roads meet one another within a relatively short distance.
Hence, in particular fields, the use of xe2x80x9ccompound navigationxe2x80x9d has taken over, where the position of the vehicle is determined by ascertaining the direction and distance from a starting point. The current position of the vehicle is thus ascertained by a type of vector addition. Such a method is also called dead reckoning. However, in this method, errors which arise in the case of any vector formation add up, which means that there also is the risk of not being able to determine the position of the vehicle with the desired degree of accuracy.
For direction determination, gyroscopes are used in the known case. Gyroscopes produce an output signal for which the term xe2x80x9cdriftxe2x80x9d has been introduced when the vehicle is traveling in a straight line or is at a standstill (no change in direction). When the vehicle changes direction, a change in the output signal is produced which is dependent on the angular speed xcfx89 of the vehicle. By integrating this signal over time, the change in direction can be ascertained.
A gyroscope can have, by way of example, an output voltage range of between 0 and 5 V, with an output voltage (drift) of 2.5 V being output in the neutral position, i.e., when the vehicle is traveling in a straight line or is at a standstill. The change in the output signal on the basis of the angular speed, i.e., mV/(xc2x0/s), is called the sensitivity.
To date, a fixed value has been assumed for the sensitivity, for example, 25 mV for 1xc2x0/sec. Although this assumption was correct in many cases, it is possible to observe that the sensitivity is subject to relatively great variation for different gyroscopes and is also dependent on the inclination of the gyroscope on the basis of the installation position. In this context, the variation range can be above 10%.
It is easy to see that ascertainment of the direction becomes relatively inaccurate when the vehicle has passed through a plurality of circles on the entrance ramp to a multistory car park, i.e., has executed rotations through a multiple of 360xc2x0. A directional error of 90xc2x0 can easily arise in this case.
The invention is based on the object of being able to determine directions more accurately. This object is achieved for a method of the type mentioned above by virtue of the feature that the result of the integration is corrected by the following steps:
a) The absolute direction of the vehicle at a first instant is determined using external auxiliary means;
b) The absolute direction of the vehicle at a second instant is determined using external auxiliary means;
c) The difference between the absolute directions is ascertained; and
d) The result of the integration is normalized to this difference.
In this case, steps a-c do not need to be carried out for each change in direction. External auxiliary means are to be understood as meaning those which have no influence on the gyroscope or on a comparable unit and are not influenced by the gyroscope. The difference between the two absolute directions can be determined relatively accurately. If the sensitivity is correct, the result of the integration ought to correspond to this difference in direction. If this is not the case, a correction variable can be formed, for example, by multiplying the correction variable used hitherto by the quotient of the difference between the absolute directions and the integration result. Such calibration can be performed during the journey, since the vehicle changes direction relatively often in this case. This also makes it possible to allow for influencing factors which change the sensitivity during the journey. Calibration can be performed using relatively few additional steps, which means that the processing complexity for the navigation system remains low. In addition, the advantage is obtained that this calibration also makes it possible to allow for the influence of the installation position of the gyroscope on its output signal.
The external auxiliary means used are preferably satellites. Although position determination using satellites is relatively inaccurate, as indicated above, direction determination using satellites is much more accurate. The signal from the navigation satellites is usually evaluated at an interval of one second, that is to say at a frequency of 1 Hz. It can thus be assumed that the systematic position determination error has not essentially changed in this short period of time, which means that the error is eliminated when a difference is formed between two positions. The direction with respect to the satellites can also be determined relatively easily by means of the Doppler effect.
As an alternative or in addition to this, the external auxiliary means used can be roads on which the vehicle is depicted. This procedure, i.e., the depiction, is also known as map matching. In this context, it is assumed that a vehicle will, with a relatively high degree of probability, be moving on a road situated close to positions which, by way of example, have been ascertained by compound navigation or using satellites. In many cases, this procedure provides sufficiently accurate statements, at least for direction determination, because the direction of the road is fixed.
The roads are preferably stored in a database. This simplifies evaluation using a computer which is required anyway for evaluating the other data arising during navigation.
The correction variable is advantageously ascertained only if the difference between the absolute directions is at least 60xc2x0. In this case, the result of the integration is large enough for, by way of example, an error no longer to be of any relatively great significance. Direction changes with a difference of at least 60xc2x0 arise when the vehicle turns at right angles, for example. Relatively large direction changes arise, for example, when the vehicle wants to turn left at a highway interchange. Since it is not possible to cross the opposite side of the highway, the vehicle usually needs to change direction through 270xc2x0 in this case.
Preferably, the correction variable is ascertained only if a predetermined average angular speed is not undershot in the period of time between the first instant and the second instant. By way of example, calibration is interrupted if the average angular speed is lower than 1xc2x0/sec. In this case, the risk of drift discrepancies having a negative effect on ascertainment of the correction variable is reduced.
The correction variable is preferably filtered. The influence of a currently ascertained correction variable is then always put in a relatively large context, so that xe2x80x9coutliersxe2x80x9d, for example, do not have too negative an effect. In the case of such filtering, it is also possible to ensure that correction variables ascertained after a large change in direction are given greater weight than correction variables ascertained after a small change in direction.